The invention relates to a semiconductor laser having a substrate region of a first conductivity type provided with a connection conductor, a first passive layer disposed thereon of the first conductivity type, a strip-shaped region located within a resonant cavity and comprising at least one active layer disposed on the first passive layer, a second passive layer disposed thereon of the second opposite conductivity type, a pn junction by which at a sufficient current strength in the forward direction coherent electromagnetic radiation can be produced, and a boundary region which is provided on either side of, but not on the strip-shaped region and comprising at least one blocking layer of the second conductivity type, which laterally bounds the strip-shaped region, the passive layers and the blocking layer having a larger band gap and a smaller refractive index for the radiation produced than the active layer and an upper layer of the second conductivity type electrically connected to a connection conductor being provided on the second passive layer and the boundary region.
A semiconductor laser of the kind described is known from European Patent Application published under No. 83697 A1.
Semiconductor lasers having a strip-shaped active region generally have the property that the electromagnetic waves amplified in the active region can oscillate in different oscillation modes. As far as wave components are concerned which propagate in the longitudinal direction of the region, these modes are designated as longitudinal modes, while for wave components having a direction of propagation in the direction of thickness these modes are designated as transversal modes and for wave components propagating in the direction of width of the active region these modes are designated as lateral modes.
For many applications, more particularly for telecommunication purposes, it is desirable that the number of possible modes of oscillation is limited as far as possible so that preferably the laser amplification is sufficient to maintain the oscillation for only one mode of oscillation.
For the transversal modes, this can be achieved by an efficient choice of the variation of the refractive index in the direction of thickness, and of the thickness of the active region. For the longitudinal modes, the object aimed at can be achieved by a suitable choice of the reflection elements defining the resonant cavity.
For the lateral modes, attempts have been made to achieve this by a suitable choice of the variation of the refractive index in the direction of width of the active region, while moreover both an optical and an electrical confinement of the active laser region is obtained. This is achieved in a structure comprising a "buried" active layer, such as that described in the aforementioned Application EP 83697.